It's not a feature of the product. It's a technical thing about the junk you've decided to attach to your server. The USB stuff has deficiencies. The developers have decided that it isn't a problem for the paying customers who fund the development of the product. It's the same reason lots of other hardware doesn't work or doesn't work well with FreeBSD and Linux.
Good. So why is it a problem to connect an SSD drive that is connected to a USB connector than when it is connected via a SATA port?
I thought this functionality (notifying the user that they are using a USB device as bootable) is there to warn the user that if the user is booting from "USB Flash" storage then the OS (TrueNAS) may crash due to USB Flash wear .
I am trying to point out that it does not matter whether the SSD disk (not USB Flash) is connected via a SATA interface cable, or whether it is connected as an external SSD HDD to USB.
The problem I am describing is that even though I have a USB SDD Drive connected, TrueNAS tells me that this is a problem because the USB should not be used to boot the TrueNAS system. And I was wondering why this is a problem... when I came here to the forum, I learned that it is because people install TrueNAS from USB Flash into the USB, which can cause the risk that the USB Flash will stop working and people will no longer start TrueNAS . Which I understand. I don't see a problem with that. But I don't have USB Flash, but I have a USB SSD disk, which I would normally have connected to a SATA cable, but I can't, because all the SATA positions are already occupied and the manufacturer of my server has a USB port not only as an output from the case, but also has one directly on the base board precisely for the purpose of being able to connect an external disk (Solid state drive, not USB Flash disk) from which to start the operating system.
And I use that one. But TrueNAS gives me a notification that it's a problem (probably because it thinks I'm using USB Flash but I'm not using it because I'm using a USB SSD (the external box that stores the Solid State Drive and I use it in USB but for the life of the drive it would should have had no effect)). TrueNAS (Scale) in terminal is normally able to identify this SSD in the USB port as an SSD for me.
Why is it a problem that I use an SSD drive to boot TrueNAS from the USB port and not from the SATA port?
Do I have to go and buy a new server immediately, because in the next versions of TrueNAS even installation on an external USB SSD will be prohibited, and will I get a discount for buying a series device from TrueNAS? etc., ... I don't know what to expect in the future so that I can go and rob a bank (joke).
I don't know the reason for the warning and I didn't find an answer to it in the documentation either, so I'm interested. And from what I found on the forum, it doesn't make much sense to me.
And... I don't know why it bothers anyone that I asked here and wrote to the developers that they should look into it. Are there religious fanatics and consider the developers as gods that are forbidden to approach or talk about? :D